AutoMod: fix RuleTarget matches not being cleared

A RuleTarget's matches dict (which stores the results of the search
checks on that item) was only being cleared in the __init__ function, so
it was actually being retained across checks against multiple different
items. This meant that {{match}} placeholders would occasionally be
replaced by the result of a match from an entirely different item. This
change makes it so that the matches will be cleared at the start of each
check instead.

r2/r2/lib/automoderator.py

@@ -887,6 +887,7 @@ def check_match_patterns(self, item, data):
         if len(self.match_patterns) == 0:
             return True
 
+        self.matches = {}
         match = None
         checked_anything = False
         for key, match_pattern in self.match_patterns.iteritems():